778 



ZOOLOGY 



tions, which give the tentacle a transversely ridged character. 

 There are no suckers : but the ridged surfaces enable the tentacle 

 to adhere firmly to -rough objects. The tentacles are arranged in 

 two series, an outer and an inner. The outer, which are borne on 

 an annular muscular ridge of the foot, are nineteen on each 

 side in both sexes. Anteriorly this muscular ridge is thickened to 

 form a massive lobe — the hood (Figs. 675, 676, hd.) — in which 

 there is a concavity for the reception of the coil of the shell. The 



Pig. 675. — Nautilus pompilius, diagrammatic lateral view of a female specimen enclosed 

 in its shell, cart, cartilage ; cten. ctenidia ; hd. hood ; inf. funnel ; jaws, jaws ; maiit. mantle ; 

 mant'. dorsal mantle fold overlapping the coil of the shell ; mus. position of lateral mass of 

 muscle : nid. nidamental glands ; sept, first septum ; sij>k. siphuncle. (After Keferstein.) 



hood bears two tentacles, and has the appearance of being com- 

 posed of the immensely developed sheat,hs of these, completely 

 fused together in the middle line : on each side the enlarged 

 sheaths of a second pair of tentacles are closely applied to, though 

 not completely coalescent with, the hood, being separated from the 

 latter by a narrow groove. The hood, with these two enlarged 

 sheaths, is covered with a thickened tuberculated skin, and acts 

 after the manner of an operculum for protecting the tentacles 



